in js/reveal.js [3368:3393]
function getSlideBackground( x, y ) {
// When printing to PDF the slide backgrounds are nested
// inside of the slides
if( isPrintingPDF() ) {
var slide = getSlide( x, y );
if( slide ) {
var background = slide.querySelector( '.slide-background' );
if( background && background.parentNode === slide ) {
return background;
}
}
return undefined;
}
var horizontalBackground = dom.wrapper.querySelectorAll( '.backgrounds>.slide-background' )[ x ];
var verticalBackgrounds = horizontalBackground && horizontalBackground.querySelectorAll( '.slide-background' );
if( verticalBackgrounds && verticalBackgrounds.length && typeof y === 'number' ) {
return verticalBackgrounds ? verticalBackgrounds[ y ] : undefined;
}
return horizontalBackground;
}